Days after being anointed the 49ers starting quarterback, Shaun Hill suffered a back injury that could keep him on the sideline this weekend.

Hill tried to practice Wednesday, but a lower-back strain proved worse than expected. Hill threw just one pass — nearly intercepted by Manny Lawson — before calling it a wrap.

With Hill and Alex Smith (sprained thumb) on the shelf, rookie Nate Davis and veteran Damon Huard shared quarterback duties for the remainder of the double-day practice.

Those two might be the only quarterbacks healthy enough to face the Cowboys on Saturday.

Could Davis or Huard start in Dallas?

"I don't know," coach Mike Singletary said. "I'm assuming (Hill) is going to play, but I don't know that. So, I don't even want to think about all the other stuff."

As Hill jogged toward the locker room after practice he flashed a thumbs-up signal to reporters and said he would "definitely" play against the Cowboys.

Singletary was less concrete, saying the injury would be evaluated on a day-to-day basis. He said Hill's lower-back strain — described as a "tweak" — was probably not serious enough to merit an MRI.

Hill first felt discomfort after he exerted himself in practice Tuesday.

"Today, it was still there," Singletary said. "He probably could have pushed himself through practice, but I told him, 'You don't need to do that. Let the other guys get the reps and let's go.'"‰"

Davis, a fifth-round pick from Ball State, played well against the Raiders last week, connecting on 6 of 11 passes for 67 yards and a touchdown. He led two touchdown drives in the 49ers' 21-20 victory.

The rookie looked solid in practice Wednesday. In his chance to run the two-minute drill, he needed just one play: Davis connected with tight end Delanie Walker for a 70-yard touchdown.

Huard, a 13th-year veteran, went 7-of-9 for 98 yards in the 49ers' exhibition opener against Denver but did not play against the Raiders.

Asked if he might keep all four quarterbacks on the roster, Singletary said: "I don't want to go there."

For any other exhibition, the 49ers would lean toward caution and have Hill rest up. But the third exhibition represents the final dress rehearsal for the NFL regular-season with first units playing as much as a half.

With Hill finally established as the starting quarterback, the Cowboys game was to be an extended chance with the starting offense.

Hill threw just nine passes over the 49ers' first two exhibitions. Smith, who has been ruled out for the Dallas game, has thrown a total of 16 passes.

In all, 49ers quarterbacks have thrown the ball just 45 times — the fewest in the league.

But Singletary said Hill's injury was no big deal.

"It's not unsettling at all, not one bit. We're going to go to Dallas and we're going to play football," Singletary said. "I'm not going to worry about who's not ready to go and who's hurt and all of those things. No. We're going to go and we should execute just fine."

Hill, 29, missed a game because of a lower-back problem in 2007. He had a small fracture in his back that kept him out of the season finale against the Cleveland Browns. The team said the injury is not related.

  • ProFootballTalk.com, citing sources, wrote that "we're hearing that (agent Eugene) Parker is dealing with the (Michael) Crabtree conundrum on the recruiting trail by blaming the holdout on the player."

    The Web site said Parker is pointing to the deal he brokered this year between the Chiefs and No. 3 pick Tyson Jackson as evidence that he is "willing and able to negotiate a fair deal for a first-round pick."

    Singletary declined to expound on Crabtree negotiations Wednesday, saying the 49ers can only worry about "the guys who we can see and evaluate every day."

  • The starting right cornerback spot remains up for grabs heading into the Dallas game. Dre Bly and third-year man Tarell Brown are the top contenders. "Still a competition," Singletary said.

    For more on the 49ers, see Daniel Brown's Hot Read blog at blogs.mercurynews.com/49ers.